The disclosure relates generally to lubrication of mating surfaces subject to relative motion and loading. More specifically, the disclosure relates to hydrodynamic lubrication of mating surfaces subject to reciprocating motion under load.
When two solid surfaces in mechanical contact slide relative to each other, the solid surfaces are subject to wear due to dry friction. When the solid surfaces are bearing surfaces, it is desirable to minimize the dry friction between the surfaces in order to prevent premature failure of the bearing due to surface wear. Lubrication is a common technique for reducing dry friction between two mating surfaces subject to relative motion. Fluid film bearings rely on a thin film of lubricant interposed between the mating surfaces to create clearance between the mating surfaces. In these bearings, dry friction between the mating surfaces is essentially replaced with fluid friction between layers of the lubricating film, and the load applied to the bearing will be supported by the viscous forces in the lubricating film.
Fluid film bearings may be hydrostatic bearings or hydrodynamic bearings. Hydrostatic bearings use an external source of pressurized fluid to force lubricant between the mating surfaces. Hydrostatic bearings have a load capacity and direct stiffness that do not depend on relative motion between the mating surfaces. In contrast, hydrodynamic bearings use relative motion between the mating surfaces to maintain a lubricating fluid film between the mating surfaces. The most basic hydrodynamic bearing is a journal bearing in which a journal pulls a converging wedge of lubricant between the journal and the bearing. The wedge forms between the journal and bearing due to the journal being eccentric with the bearing as the journal rotates. As the journal gains velocity, liquid flows between the two surfaces at a greater rate. The lubricant, because of its velocity, produces a liquid pressure in the lubricant wedge that is sufficient to keep the two surfaces separated while supporting the applied load.
True hydrodynamic lubrication is difficult to achieve between mating surfaces operating under reciprocating motion and load. The reciprocating motion will prevent formation of a coherent fluid film in the interface between the mating surfaces. Without a coherent fluid film between the mating surfaces, boundary lubrication will prevail over hydrodynamic lubrication during normal operation. In the boundary lubrication mode, the mating surfaces will be partially separated by fluid and partially in mechanical contact. Such mechanical contact, when coupled with high contact pressure between the mating surfaces, will accelerate wear of the mating surfaces compared to the case where full-film hydrodynamic lubrication develops and is maintained during normal operation.